1. Field of Invention
The present invention relates, inter alia, to a driver circuit. One particular application of such a driver circuit is for driving a pixel of an organic electroluminescent device.
2. Desription of Related Art
An organic electro-luminescent (OEL) element comprises a light emitting material layer sandwiched between an anode layer and a cathode layer. Electrically, this element operates like a diode. Optically, it emits light when forward biased and the intensity of the emission increases with the forward bias current. It is possible to construct a display panel with a matrix of OEL elements fabricated on a transparent substrate and with at least one of the electrode layers being transparent. One can also integrate the driving circuit on the same panel by using low temperature polysilicon thin film transistor (TFT) technology.
In a basic analog driving scheme for an active matrix OEL display, a minimum of two transistors are required per pixel (FIG. 1): T1 is for addressing the pixel and T2 is for converting the data voltage signal into current which drives the OEL element at a designated brightness. The data signal is stored by the storage capacitor Cstorage when the pixel is not addressed Although p-channel TFTs are shown in the figures, the same principle can also be applied for a circuit with n-channel TFTs.
There are problems associated with TFT analog circuits and OEL elements do not act like perfect diodes. The light emitting material does, however, have relatively uniform characteristics. Due to the nature of the TFT fabrication technique, spatial variation of the TFT characteristics exists over the entire panel. One of the most important considerations in a TFT analog circuit is the variation of threshold voltage, ΔVT, from device to device. The effect of such variation in an OEL display, exacerbated by the non perfect diode behaviour, is the non-uniform pixel brightness over the display panel, which seriously effects the image quality. Therefore, a built-in circuit for compensating a dispersion of transistor characteristics is required.
A circuit shown in FIG. 2 is proposed as one of built-in for compensating a variation of transistor characteristics. In this circuit T1 is for addressing the pixel. T2 operates as an analog current control to provide the driving current T3 connects between the drain and gate of T2 and toggles T2 to be either a diode or in saturation. T4 acts as a switch. Either T1 or T4 can be ON at any one time. Initially, T1 and T3 are OFF, and T4 is ON. When T4 is OFF, T1 and T3 are ON, and a curt of known value is allowed to flow into the OEL element, through T2. This is the programming stage because the threshold voltage of T2 is measured with T2 operating as a diode (with T3 turned ON) while the programming current is allowed to flow through T1, through T2 and into the OEL element. T3 shorts the drain and gate of T2 and turns T2 in to a diode. The detected threshold voltage of T2 is stored by the capacitor C1 connected between the gate and source terminals of T2 when T3 and T1 are switched OFF. Then T4 is turned ON, the current is now provided by VDD. If the slope of the output characteristics were flat, the reproduced current would be the same as the programed current for any threshold voltage of T2 detected. By turning ON T4, the drain-source voltage of T2 is pulled up, so a flat output characteristic will keep the reproduced current the same as the programmed current. Note that ΔVT2 shown in FIG. 2 is imaginary, not real.
A constant current is provided, in theory, during the active programming stage, which is t2 to t5 in the timing diagram shown in FIG. 2. The reproduction stage starts at t6.
The circuit of FIG. 2 is advantageous but there is an on-going desire to reduce power consumption. In particular, implementation of the current-source in the circuit of FIG. 2 requires a bias voltage (VBIAS) in addition to the supply voltage (VDD). Although the supply voltage (VDD) could be increased to cover the required bias voltage (VBIAS)—which would have the advantage of reducing the component count, there is still an overall increase in system power consumption to program with any value of data current (IDAT).
Attention is, by the present invention, drawn to the fact that all currents passing through the circuit of FIG. 2 pass through the OEL element. The significance of this to the present invention will be apparent from the description given hereinafter.